1. Field of the Invention
The invention relates generally to the field of network synchronization. More particularly, an embodiment of the invention relates to methods of and/or apparatus for providing a two-way timing protocol.
2. Discussion of the Related Art
Referring to FIG. 1A, the legacy interoffice distribution approach utilized two external synchronization references from equal or higher stratum offices providing traceability back to a sparse set of stratum 1 offices. This tree topology introduces multiple transmission links, network element clocks and Building Integrated Timing Supplies (BITS) into a primary and secondary path. A myriad of issues jeopardize the performance of this “send and pray” distribution approach.
First, the path quality is limited by the “weakest link”. One poorly performing clock could degrade the entire downstream path. Recent improvement in Stratum 3E oscillator screening have shown that over 10% of the older systems could have bad actor oscillators, and that this problem is likely pervasive in telephony. Frequent transmission path switching can severely degrade a clock even though it reports normal operation. Also, the wide distribution in oscillator performance from like unit to like unit, results in some level of bad acting oscillators in the network. Simplistic synchronization messaging schemes (SONET/SDH) do not adequate address the marginal performance issues.
Second, the distribution is inherently non-traceable. The principle stratum distribution rule of only receiving timing from clocks of equal or higher stratum level cannot be guaranteed. Even more insidious is the danger of receiving timing from a path that already includes the “receiving clock”. Known as a timing loop, this can occur as simply as setting the “near/far” timing mode of network element such as a echo canceller wrong so that timing is send back along the same path. Timing loops generate slowly increasing degradation and are difficult to detect and resolve.
Third, path diversity cannot be assured. Shared resources such as clocks, routes, cables, and offices along two supposedly independent paths to a stratum 1 source are common. The interdependence of a primary and secondary reference will not only reduce reference availability, but can aggravate performance problems as clocks continuously switch reference inputs.
Referring to FIG. 1B, the interoffice synchronization distribution problems discussed above were the impetus for the trend to more PRS (primary reference standard) systems in the network. A properly designed PRS provides direct traceability to UTC time and serves as a firewall protecting the Central Office from external synchronization “viruses”. Each generation of PRS systems permits more cost effective utilization in the network.
The Bell System Reference Frequency Supply is characteristic of the original PRS approach. The design was based on multiple primary clocks, a voting system and a UTC monitoring system. The system was expensive at approximately $250,000 at the time and, therefore, designed for limited application. In fact, the original slip rate standard (G.822) only anticipates a maximum of 14 such “plesiochronous stratum 1 systems” in an international end-to-end connection.
The next step in the evolution of PRS architecture was the introduction of UTC (universal time coordinated) traceable system utilizing non-cesium oscillators as flywheels. This approach gained credence in the standards with the introduction of the term Primary Reference Clock (PRC) or equivalently Primary Reference Source (PRS). ANSI T10.101 was the first standard to define a PRC (PRS) as a UTC traceable clock using either Cesium or other local oscillators. AT&T replaced the BSRF Supply with 18 regional PRS systems using GPS and Rubidium coincident with its upgrade to a digital fiber backbone. Sprint and MCI used similar systems based on Loran-C traceability. The systems were not tightly integrated and required costly installation of rooftop antenna and cable systems. More refinements on this approach such as re-utilization of existing TSG oscillators and designs to extend antenna cable distance permitted the penetration of the PRS into the core backbone networks during the 90s. The current generation PRS in systems such as the TS3xxx and OT-21, have eliminated the need for a rooftop antenna system in many applications. However, the systems still require an antenna system mounting externally on an outside wall or on certain windows. The need for special antenna installation although significantly reduced is not eliminated. However, this reduction is permitting the introduction of PRS system in many more central offices. What is needed is an approach that will permit the introduction of PRS systems in all central offices, and inter-office, and also intra-office (e.g., with a customer premises).
A time transfer protocol can be operated at various layers of a communication protocol. For example, many are familiar with Network Timing Protocol (NTP). The NTP protocol is commonly used in an IP network to establish a best effort transfer of time of day. Although NTP attempts to determine transport delay using a two-way approach, the performance is limited by the lack of reciprocity in path latency on each direction. This is a basic limitation of supporting a datagram type routing through a network with dynamic loads. As are result while NTP can typically support relatively coarse millisecond time accuracies, it has no capability of assuring even this level of accuracy, much less anything precise.
In an attempt to address the deficiencies of NTP, there have been previous efforts to constrain the path delay. For example a layer 2 based protocol would theoretically substantially eliminate the network routing effects. One example of such a protocol is the recently adopted IEEE 1588™ precise timing protocol. The primary advance is that 1588™ is constrained to be non-routed. Packet time stamping is to be performed as close to the physical layer as possible, and layer 2 switching between clocks is anticipated. This standard supports up to one two-way transaction per second and is capable of better than 1 microsecond timing performance.
Heretofore, the requirement(s) of obviating the need for all external GPS antennas, enabling the installation of economical PRS systems in all offices, providing greater accuracy and precision and ensuring compatibility with legacy balance of plant interconnects have not been fully met. What is needed is a solution that simultaneously solves all of these problems.